class_docstr: Eckerle4 objective function.
  Eckerle, K., NIST (1979).
  Circular Interference Transmittance Study.
  
  ..[1] https://www.itl.nist.gov/div898/strd/nls/data/eckerle4.shtml
  
  #TODO, this is a NIST regression standard dataset, docstring needs
  improving
code: def fun(self, x, *args):
      self.nfev += 1
  
      vec = x[0] / x[1] * exp(-((self.b - x[2]) ** 2) / (2 * x[1] ** 2))
      return sum((self.a - vec) ** 2)
  
imports: from numpy import abs, asarray, cos, exp, arange, pi, sin, sqrt, sum
  from .go_benchmark import Benchmark
label_desc: Use image node_id 2 for calling the Eckerle4 obj's underlying member method code with example usage: obj.fun(x) and returns: sum